1. Field of the Invention
The present invention relates to polyester polyols of low molar mass, their preparation and use in coating compositions.
2. Description of Related Art
Presently used two-component polyurethane coating materials are based on hydroxy-functional polyesters or polyacrylates, on acrylic-modified polyesters or polyester-modified polyacrylates, or mixtures of these resins, and are more environmentally compatible than their prior art counterparts. This is because presently used two component polyurethane coating materials generally require less solvent for dilution, thus enabling them to be formulated to give a ready-to-spray transparent or pigmented coating material or surfacer. These low-solvent or high-solids coating materials can only be prepared because the hydroxy-functional polymers employed (also referred to as binders), still exhibit sufficiently good physical initial drying despite their low degree of polymerization.
In order to have sufficient initial drying (i.e. short time span until the applied coating is no longer tacky) properties, these low molar mass polyols must comprise rigid chain units, which initially reduces the solution viscosity and at the same time leads to glass transition temperatures (T.sub.g) of at least 20.degree. C. It is precisely at very low degrees of polymerization that the dependence of the glass transition temperature on the degree of polymerization is critical, and predictions made on the basis of additivity equations which link the mass fraction or weighted molar proportions of the units with the T.sub.g are generally not valid. One is forced to look for suitable units on a purely empirical basis which will then, it is hoped, lead to a low molar mass polymer having the desired properties described above.
Examples of acrylate polyols which, in pure form or as mixtures, generally meet these requirements are described for example in EP-A 0 638 591 and EP-A 0 741 149. Coating compositions based on these branched polymers are notable for good weathering stability, high solids contents, rapid drying, a long pot life (at least 8 hours) and, in particular, the possibility of universal application in intermediate coats and in pigmented and unpigmented systems.
Examples of polyester polyols that have aliphatic branched and monocyclic units are also described in EP-A 0 705 858. They are notable for good weathering stability, high solids contents, good elasticity, and rapid drying. However, they have disadvantages in terms of their adhesion to various substrates, such as base coat in automotive refinish, and various metallic substrates (cold-rolled sheet iron).
Surprisingly it has now been found that two-component polyurethane coating materials which comprise, as a hydroxy-functional component, low molar mass polyester polyols of the present invention, which are based on selected starting materials and are described in more detail below, possess one or more of certain favorable properties including those referred to above and, moreover, exhibit good adhesion to base coat in automotive refinishes and to various metallic substrates.